Film Density and Surface Area

The presence of porosity in the columnar structure, closed and open voids, and pinholes means that the film density is lower than that of the bulk material and that the surface area exposed to the ambient is much higher than the geometrical surface area. This low density and high surface area influence many film properties, such as hardness, deformation, chemical etch rate, resistivity, and index of refraction.

Open porosity features pores that connect to the free surface and contribute to the surface area of the film. The coalescence of lattice defects along surfaces such as grain boundaries and the open space between the columns in the columnar microstructure are sources of such porosity. Process parameters that affect the growth of the columnar microstructure affect the film porosity. For example, the porosity of vacuum-deposited films can be varied by controlling the substrate surface roughness, angle of incidence of the adatom flux, or the gas pressure during deposition.

Through porosity is a special case of open porosity where the porosity is aligned in such a manner as to allow an open space to exist between the film surface and the substrate surface. Through porosity develops from the columnar growth, particularly on rough surfaces, and the growth on surface contaminants such as particulates (see Fig. 6). Such porosity causes pinholes in the film and allows corrosion of the substrate through the pinholes, electrical "opens" in patterned electrical circuits, optical transmission, and high surface diffusion rates from the interface to the film surface.

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